1. Technical Field
The present invention generally relates to light emitting devices and, particularly, to a solid-state light emitting device with high light extraction efficiency.
2. Description of Related Art
Referring to FIGS. 1 and 2, a typical solid-state light emitting device, e.g., a semiconductor light emitting diode (LED) 600 is illustrated. The LED 600 includes an insulating substrate 610, a light emitting structure 620 formed on the insulating substrate 610, an N-electrode 630 and a P-electrode 640. The light emitting structure 620 includes an N-type layer 621, a P-type layer 623 and an active layer 622 sandwiched therebetween. The N-type layer 621 has an exposed portion uncovered by the active layer 622 and the P-type layer 623. The N-electrode 630 is an N-contact pad and formed on the exposed portion of the N-type layer 621. The P-electrode 640 is a P-contact pad and arranged on the P-type layer 623. The N-electrode 630 and the P-electrode 640 are diagonally arranged with each other. However, due to a relatively high resistance of the P-type layer 623, the current injected at the P-electrode 640 does not spread laterally away from the P-electrode 640 and does not spread laterally within the P-type layer 623. The current flows from the P-electrode 640 along the shortest path (i.e., usually vertically) across the active layer 622 to the N-type layer 621. As a result, an area utilization ratio of the active layer 622 is lowered and thus the light extraction efficiency of the light emitting diode 600, especially with regard to a light emitting diode with a large sized light emitting structure, is degraded.
Referring to FIG. 3, in order to overcome the above-mentioned issue associated with the low area utilization ratio of the active layer, an improved light emitting diode 700 is disclosed in U.S. Pat. No. 6,307,218 B1, entitled “electrode structures for light emitting devices”. The light emitting diode 700 includes a light emitting structure 720 having an N-type layer 721 and a P-type layer 723, a comb-shaped N-electrode 730 and a comb-shaped P-electrode 740. The comb-shaped N-electrode 730 and P-electrode 740 are interdigitated with each other and respectively ohmic contacted with the N-type layer 721 and the P-type layer 723. The use of the comb-shaped N-electrode 730 and P-electrode 740 can macroscopically improve current spreading to some degree, but still subject to current crowding in the microscopical level of the comb-shaped P-electrode 740. Accordingly, the light extraction efficiency of the light emitting diode 700 is low.
Therefore, what is needed is a solid-state light emitting device with good current spreading and thus relatively high light extraction efficiency.